1. Field of the Invention
The present invention relates to a surface profile measuring instrument and a surface profile measuring method. Specifically, it relates to a surface profile measuring instrument and surface profile measuring method for continuously autonomic-measuring a surface profile of a workpiece by scanning and profiling the surface of the workpiece by a probe.
2. Description of Related Art
A surface profile measuring instrument that measures a profile of a surface of a workpiece with a contact probe attached to a coordinates measuring machine has been known, the instrument continuously measuring the surface profile of a workpiece by scanning and profiling the surface of a workpiece with a tip end of a stylus.
As shown in FIG. 14, a conventional surface profile measuring instrument 1 has a contact probe 2 having a contact portion 22 to be in contact with a surface of a workpiece W, a drive mechanism 3 for moving the contact probe 2 in X, Y and Z directions, and a controller 4 that controls the movement of the contact portion 22 of the contact probe 2 through the drive mechanism 3.
As shown in FIG. 15, the contact probe 2 has a stylus 21 having the contact portion 22 at a tip end thereof, a stylus holder 23 that holds the stylus 21, a vibrator provided on the stylus holder 23 for vibrating the stylus 21 in the axial direction, and a vibration detector 27 that detects the variation in the vibration of the stylus 21 to output a detection signal. The contact portion 22 is a sphere having radius r.
The drive mechanism 3 uses a slide mechanism capable of slide movement in X, Y and Z directions used in conventional coordinates measuring machine. For the convenience of description, Z-axis is set parallel to the stylus 21 in FIG. 14. Further, X and Y-axes are set on a plane orthogonal with the stylus 21. The X, Y and Z slide mechanisms respectively have a linear encoder (not shown) for measuring drive amount in respective directions.
The controller 4 has a scanning mechanism for moving the contact portion 22 while scanning along workpiece surface so that the detection signal from the vibration detector 27 becomes a predetermined reference signal value, and a profile processor for calculating the surface profile of the workpiece based on the position information of the contact portion 22 when the detection signal from the vibration detector 27 becomes the reference signal value.
In thus arranged surface profile measuring instrument 1, surface profile of a plane parallel to Z-axis is measured on X-Y plane as shown in FIG. 16.
The contact portion 22 is moved along the surface of a workpiece W according to the command of the scanning mechanism. The drive amount of the drive mechanism in X, Y and Z directions is measured by the linear encoder to obtain the position information of the contact portion 22.
When the contact portion 22 is moved in a scanning manner, the detection signal varies in accordance with relative position between the contact portion 22 and the surface of the workpiece. The positional relationship between the contact portion 22 and the workpiece surface and detection signal varying in accordance with the positional relationship are shown in FIG. 17(D).
When the contact portion 22 is remote from the workpiece W as shown in FIG. 17(A), the vibration of the contact portion 22 is not restricted and the detection signal becomes the maximum. A pressing force is applied to the contact portion 22 when the probe 2 is moved by the drive mechanism 3 and the stylus 21 is bent as shown in FIG. 17(C) after the contact portion 22 comes into contact with the workpiece surface as shown in FIG. 17(B).
The vibration of the contact portion 22 is restricted by the pressing force applied by the workpiece W to decrease the level of the detection signal. The detection signal varies according to sensitivity characteristics dependent on the pressing force applied from the workpiece surface to the contact portion 22. The detection signal reaches a reference signal value when the flexure (retraction d) of the stylus becomes a predetermined value as shown in FIG. 17(C).
The drive amounts of the drive mechanism in X, Y and Z-axes are sampled when the detection signal reaches the reference signal value. Then, the position information of the contact portion 22 when the detection signal reaches the reference signal value can be obtained.
The surface profile of the workpiece W can be measured by profile processing using the position information of the contact portion by the profile processor.
The example shown in FIG. 16 will be schematically described using FIG. 20. FIG. 20 represents output of the detection signal. In the present example, it is assumed that the measurement surface of the workpiece W is parallel to Z-axis.
When measurement start command is sent to the controller 4, surface profile measurement is started.
Initially, the contact portion 22 is moved from a point P0 where the contact portion 22 is not in contact with the surface of the workpiece W in a direction to be brought into contact with the workpiece according to an approach vector (step 1). In the present example, the approach direction is aligned with the Y-axis.
After step 1, the contact portion 22 is in contact with the measurement surface of the workpiece W and the detection signal reaches the reference position signal value (reference signal value) at point P1.
When the detection signal reaches the reference signal value, the contact portion 22 is moved in accordance with a prior-movement vector having a predetermined orientation relative to the approach vector (in perpendicular direction in the present instance) (step 2).
At this time, the detection signal varies when the contact portion 22 is forcibly pressed to the workpiece W on account of irregularity on the surface of the workpiece W (point P2).
When the detection signal varies, the contact portion 22 is moved by a correction vector having a predetermined orientation relative to the prior-movement vector (in perpendicular direction in the present instance) (step 3).
When the detection signal reaches the reference signal value by the movement of the contact portion 22 in step 3 (point P3), a scanning vector having a predetermined magnitude is generated in a direction connecting the points P1 and P3 and the contact portion 22 is moved according to the scanning vector (step 4).
Subsequently, when the contact portion 22 is moved in accordance with the correction vector having a predetermined orientation and the detection signal reaches the reference signal value (point P5), the scanning vector having a predetermined magnitude is generated in a direction connecting the points P3 and P5 and the scanning measurement of the measurement surface of the workpiece W is conducted in a similar manner.
Incidentally, the position information of the contact portion 22 when the detection signal reaches the reference signal value is a position measured by drive amount of the drive mechanism 3.
FIG. 18 shows the relationship between the contact portion 22 and the workpiece surface when the detection signal reaches the reference signal value. Solid line shows an actual position of the contact portion 22 when the contact portion touches the workpiece surface. Coordinates of the center of the contact portion 22 at the actual position is represented as x0. It is preferable that the coordinates of the center of the contact portion 22 is sampled at the actual position, however, the probe 2 has to be forcibly pressed by a predetermined retraction d before the detection signal reaches the reference signal value. Dotted line shows imaginary position of the contact portion 22 when the contact portion 22 is pressed by the retraction d relative to the actual position. The coordinates of the center of the contact portion 22 (i.e. coordinates of the center of the stylus 21) is represented as X. The coordinates X of the center of the contact portion 22 at the imaginary position is measured by the linear encoder of the drive mechanism 3 to be sampled.
In other words, the actual position x0 of the contact portion 22 when the detection signal reaches the reference signal value is located at a position where the position X measured by the encoder of the drive mechanism 3 is corrected by the retraction d, which can be represented as x0=X−d. Further, the surface profile of the workpiece to be measured is a contact point xp of the contact portion 22 with the workpiece surface, which can be represented as xp=x0+r.
Accordingly, in order to obtain the location of the contact point between the contact portion 22 and the workpiece surface (i.e. xp) based on the position information of the contact portion 22 measured by the encoder of the drive mechanism 3 (i.e. X), correction formula: xp=X+r−d, is used.
The surface profile of the workpiece W can be measured by sampling the position information of the contact portion 22 when the detection signal reaches the reference signal value and processing the position information.
As described above, when a surface parallel to the vibrating direction of the contact portion 22 is measured, the detection signal reaches the reference signal value when the probe 2 is pressed to the workpiece W by the retraction d.
However, as shown in FIG. 19 for instance, when a surface of a workpiece W having inclination relative to the vibration of the contact portion 22 is measured, the vibration of the contact portion 22 is further restricted by the workpiece surface as compared to the condition shown in FIG. 18. Then, the relationship between the detection signal and the retraction is changed according to the inclination angle. In the instance shown in FIG. 19, the detection signal reaches the reference signal value before the retraction d is reached. As a result, the position information of the contact portion 22 is sampled before reaching the retraction d. In this case, the surface profile of the workpiece cannot be correctly obtained even after correcting the position information of the contact portion 22 when the detection signal reaches the reference signal value by the constant retraction d.
Further, when the measurement surface of the workpiece W is parallel to the Z-axis (the instance shown in FIG. 20), the contact point of the contact portion 22 against the workpiece W may not be located in the direction of the correction vector on the X-Y plane passing the center of the contact portion 22, so that the surface profile of the workpiece may be correctly measured by correction with constant retraction d.
As described above, since the relationship between the detection signal and the retraction (sensitivity characteristics) varies in accordance with the angle formed by the vibration of the contact portion 22 and the measurement surface of the workpiece, the surface profile of a workpiece being inclined relative to the vibration of the contact portion 22 may not be correctly measured.
Further, even when the sensitivity characteristics does not change, when the contact point between the contact portion 22 and the workpiece W is not located in the direction of the correction vector, the contact position cannot be correctly obtained, so that the surface profile of such workpiece also cannot be obtained.
The above problem is not restricted to a vibration-type probe 2, but also applied to a profiling probe that detects the contact between the contact portion 22 and the workpiece W by measuring the distortion of the stylus 21. Specifically, the relationship between the retraction of the probe 2 and the distortion of the stylus 21 varies in accordance with the angle formed between the stylus and the workpiece surface or between the correction vector and the workpiece surface. As a result, the surface profile of the workpiece W being inclined relative to the shaft of the stylus 21 or the correction vector cannot be correctly measured.
Such problem also occurs when surface profile of a workpiece is measured with a non-contact probe. For instance, the sensor head of an electrostatic capacitance probe disclosed in Japanese Patent Laid-Open Publication No. 2001-194105 has a sensor electrode and a reference electrode arranged in a ring surrounding the sensor electrode. When the sensor electrode of such sensor head is driven by a high-frequency signal, the high-frequency signal changes in accordance with the distance between the sensor head and the measurement surface of the workpiece (i.e. electrostatic capacitance) to cause a change in terminal voltage of the sensor electrode. Accordingly, the distance gap between the sensor head and the measurement surface can be measured by detecting the terminal voltage of the sensor electrode.
Such electrostatic capacitance requires that the sensor head normally opposes to the workpiece surface (i.e. opposes to the workpiece along normal line direction of the surface) in measuring the workpiece. This is because, when the sensor head is not in normal opposing condition relative to the workpiece surface, electrostatic capacitance between the portion around the tip end of the sensor head and the measurement surface is not uniformly distributed, which results in the change in the sensitivity characteristics, so that the distance gap cannot be correctly measured. In other words, when such non-contact probe is used in a scanning measurement where the workpiece surface may not be constantly in normal-opposing condition relative to the sensor head, the surface profile of the workpiece may not be correctly measured.